Nonwoven fiber structures frequently consist of a random yet homogeneous agglomeration of long and short fibers. Long fibers are fibers of both natural and synthetic origin that are suitable for textiles. They are longer than 0.25 inches and generally range between 0.5 and 2.5 inches in length. Short fibers are suitable for papermaking and are generally less than about 0.25 inches long, such as wood pulp fibers or cotton linters. It is known in the art that strong nonwoven structures can be made by rapidly and reliably blending inexpensive short fibers with strong long fibers.
Random distribution of the blended fibers results in an isotropic web or structure that is uniformly strong in all directions. The fibers can also be directionally disposed or aligned, resulting in an anisotropic fabric that is strong in the direction of alignment. Nonwoven fabrics are less costly than woven or knitted material, yet are more or less comparable in physical properties, appearance, and weight. Thus, inexpensive nonwoven fabrics are available for a wide variety of products, including, hand towels, table napkins, sanitary napkins, hospital clothing, draperies, cosmetic pads, etc. These nonwoven webs can be particularly advantageous when formed as a layered or composite structure having selective absorbent properties.
The desired utility and characteristics of the nonwoven end product dictate the types of fibers and the relative proportions of long and short fibers in a web. The desired characteristics may include, for example, tear resistance, abrasion resistance, stretchability, strength, absorption or non-absorption to different liquids, heat sealability, and resistance to delamination. Thus, a strong yet absorbent web may advantageously be formed from two or more long and short fibers, such as rayon and wood pulp combined in varying percentages.
There are many different methods and devices useful for making nonwoven webs and other fibrous structures. Conventional carding or garnetting methods produce nonwoven fiber webs, but these are generally limited to textile length fibers.
The "Rando-Webber " process may be used to make nonwoven webs. In this process, pre-opened textile fiber material is delivered to a lickerin that opens the fibers further, and introduces them to a high-velocity low-pressure air stream. The fibers are randomly deposited on a condensing screen to form an isotropic web. While a uniform web of textile fibers can be obtained, this process is not suitable for use with short fibers or blends of long and short fibers.
U.S. Pat. No. 3,512,218 of Langdon describes two lickerins and rotary feed condenser assemblies arranged in parallel one after the other. Isotropic nonwoven webs are formed with this apparatus by feeding fibrous material to the lickerins, where the fibers are individualized and deposited on a condenser screen. A single airstream is divided into two parts and acts to doff the fibers from the lickerins and deposit them onto the screen, where the web is formed. This method cannot be used to homogeneously blend two streams of fibers.
In U.S. Pat. No. 3,535,187 of Woods there is described apparatus for producing a layered web of randomly oriented fibers joined at the interface of adjacent layers by a small zone of textile length fibers extending across the interface. Wood's device provides individualized fibers which are deposited on a pair of cylindrical condenser screens by a pair of respective lickerins acting in cooperation with high speed, turbulent air streams that move faster than the lickerin in order to doff the fibers. However, the air speed must also be controlled so that the fibers do not forcibly impact on the condensers. The condenser screens are positioned closely adjacent to one another and the layers of fibers on the condensers are compressed between the condensers to form a composite nonwoven web with some blending at the interface between layers. However, there is no substantial fiber mixing zone adjacent to the condensers, and the intermixing of fibers is minimal.
One way of making a nonwoven web consisting of a mixture of randomly oriented long and short fibers uses a milling device to individualize short fibers and a lickerin to individualize long fibers. The fibers are mixed in a mixing zone, and the mixture is deposited on a condenser to form a nonwoven web. Though randomly oriented, the mixed fibers are stratified rather than homogeneously blended. The long fibers predominate on one side of the web and the short fibers predominate on the other. In addition, undesirable clumps of fibers or "salt" occur in this web product, because the mill does not completely individualize the short wood pulp fibers.
Another method used to make webs of mixed and randomly oriented long and short fibers introduces pre-opened long and short fibers to a single lickerin for individualization. However, the optimum lickerin speeds for long and short fibers are different. To prevent the degradation of long fibers, this device must operate at the slower speed that is optimum for long fibers. As a result, the speed and throughput of the device is compromised.
Methods and devices which produce a blend of long and short fibers without clumps or salt are disclosed in U.S. Pat. No. 3,772,739 of Lovgren. Lovgren provides for the separate and simultaneous individualization of each type of fiber on separate lickerins, each operating at an optimum speed for the fiber it opens. For example, long fibers such as rayon are supplied to a lickerin operating in the neighborhood of 2400 rpm. Pulpboard is supplied to a lickerin operating in the neighborhood of 6000 rpm, a speed that would damage long fibers. The fibers are doffed from their respective lickerins by separate air streams and are entrained in the separate air streams. These streams are subsequently mixed in a mixing zone in order to blend the fibers. The homogeneous blend is then deposited in a random fashion on a condenser disposed in proximity to the mixing zone. While the Lovgren apparatus is useful, it does not lend itself to the preparation of a wide variety of webs.
Another method of producing homogeneous blends of fibers is disclosed in commonly owned U.S. Pat. No. 3,740,797 of Farrington. Farrington discloses a method and machine wherein supplies of fibers are fed to oppositely rotating parallel lickerins, which are operated at respective optimum speeds to produce individualized long and short fibers. The individualized fibers are doffed from the lickerins by centrifugal force and by high velocity air streams directed against any fibers tending to cling to the lickerin structure. The individualized fibers from each supply are entrained in their respective air streams and are impelled toward each other at high velocities along trajectories that intersect in a mixing zone, where at least a portion of the fibers from each supply may be blended. A condensing means or screen with a vacuum chamber below it communicates with the mixing zone so that the blended fibers are deposited on the condenser screen within a condensing zone to produce an isotropic web of fibers. This screen is moved in a direction, i.e. the "machine direction," which is perpendicular to the axis of the lickerins. In addition, a baffle can be interposed between the air streams to control the degree of mixing and the respective location of the long and short fibers in the composite web.
Farrington provides a method and apparatus for producing an air laid nonwoven web of homogeneously blended and randomly oriented short and long fibers that is isotropically strong and is free of salt. While Farrington provides for a wide variety of nonwoven web products, that process is still insufficient to produce many desirable nonwoven structures or webs.
Thus it would be advantageous to provide a method and apparatus for making thicker webs more rapidly than with the Farrington process, and webs having a wider range of shapes and composite structures than can be made on known machines by known methods.